1. Field of the Invention
The present invention relates to a process for forming a thin film used for an SOI substrate or a photoelectric conversion device such as a solar cell or an area sensor.
2. Related Background Art
An integrated circuit formed on a substrate having an SOI (Semiconductor On Insulator) structure has a variety of advantages in comparison with an integrated circuit formed on an ordinary Si wafer. For example, (1) dielectric isolation can be made easily to enable high integration, (2) it is excellent in radiation resistance, (3) floating capacitance can be reduced to enable high speed operation, (4) a well forming step can be omitted, (5) latch up can be prevented, and (6) since a complete depletion type electric field effect transistor can be formed by thin film formation, high speed operation and low power consumption can be attained.
As a process of producing a substrate with the SOI structure, there is a process such as disclosed in U.S. Pat. No. 5,371,037 or in Applied Physics Letters, vol. 64, p.2108 (T. Yonehara et al., Appl. Phys. Lett. vol.64, 2108 (1994)). FIGS. 22A to 22E and FIGS. 23A to 23D show the production process. In the figures, reference numerals 1 and 5 denote Si wafers; 2, a nonporous Si layer; 3, a porous Si layer; 4, an epitaxial Si layer; 6, a single crystal Si layer; and 7, an Si oxide layer. First, the Si wafer 1 that forms a device substrate is prepared as shown in FIG. 22A, which is then anodized to fabricate a substrate having the porous Si layer 3 formed on a surface of the nonporous Si layer 2 as shown in FIG. 22B. Then, the epitaxial Si layer 4 is formed on a surface of the porous Si layer 3 as shown in FIG. 22C. On the other hand, the Si wafer 5 that forms a support substrate is prepared as shown in FIG. 22D, and the surface thereof is oxidized to fabricate a substrate in which the Si oxide layer 7 is formed on a surface of the single crystal Si layer 6 as shown in FIG. 22E. Then, the substrate 2, 3 and 4 shown in FIG. 22C is turned over above the substrate 6 and 7 shown in FIG. 22E so that the epitaxial Si layer 4 and the Si oxide layer 7 are opposed to each other as shown in FIG. 23A, and both the substrates are bonded to each other with the epitaxial Si layer 4 and the Si oxide layer 7 in contact with each other, as shown FIG. 23B. Thereafter, the nonporous Si layer 2 is mechanically removed by grinding to expose the porous Si layer 3 as shown in FIG. 23C. Therefore, the porous Si layer 3 is wet etched with an etchant capable of selectively removing the porous Si layer 3 to remove the porous Si layer 3 as shown in FIG. 23D. As a result, the thickness of the epitaxial Si layer 4 which becomes a semiconductor layer on the underlying insulating layer becomes remarkably uniform.
In producing the substrate of the SOI structure, the above-described production process requires one substrate 1 every time one SOI substrate is produced because the nonporous Si layer 2 is removed by grinding during the transformation of the substrate shown in FIG. 23B into the substrate shown in FIG. 23C. Under this circumstance, there has been proposed in Japanese Patent Application Laid-Open No. 7-302889 that the nonporous Si layer 2 is used a plurality of times during the process of producing the SOI substrate. In other words, when transforming the substrate shown in FIG. 23B into the substrate shown in FIG. 23C, a process of exerting a tensile force, a squash force, a sharing force or the like on the substrate shown in FIG. 23B, of inserting a jig into the porous Si layer 3, or the like is used to separate the bonded layers 4, 7 and 6 which become an SOI substrate from the nonporous layer 2 via the porous Si layer 3. Thus, the remaining nonporous Si layer 2 is used a plurality of times as the Si wafer 1 shown in FIG. 22A.
On the other hand, although the main current of existing solar cells are those which use amorphous Si as a structure suitable for attaining a large area cell, attention has also been paid to a solar cell of single crystal Si or polycrystal Si from the standpoint of conversion efficiency and service life. Japanese Patent Application Laid-Open No. 8-213645 discloses a process of providing a thin film solar cell at a low cost. In the process, as shown in FIG. 24, a porous Si layer 3 is formed on an Si wafer 1, and then a p.sup.+ type Si layer 21, a p-type Si layer 22 and an n.sup.+ type Si layer 23 which become a solar cell layer are epitaxially grown on the porous Si layer 3. After a protective film 30 is formed on the n.sup.+ type Si layer 23, a jig 31 is made to adhere to a rear surface of the Si wafer 1 with an adhesive 34, and a jig 32 is made to adhere to a front surface of the protective film 30 with an adhesive 34. Subsequently, the jigs 31 and 32 are pulled in opposite directions as shown by P in FIG. 24 to mechanically break the porous Si layer 3, thereby separating the solar cell layers 21, 22 and 23 from the Si wafer 1. Then, the solar cell layers 21, 22 and 23 are disclosed to be interposed between two plastic substrates, thus producing a flexible thin film solar cell. It is further disclosed therein that the Si wafer 1 can be used several times. Also, there is disclosed that a partial cut 33 is formed on a side wall of the porous Si layer 3 by a mechanical process or irradiation with a laser beam prior to the application of a pulling force.
In producing an SOI substrate, the process disclosed in Japanese Patent Application Laid-Open No. 7-302889 mentioned above can reduce the cost because the Si wafer is used several times. However, this process is not that sufficient in actual operation.
On the other hand, according to the production process as disclosed in Japanese Patent Application Laid-Open No. 8-213645 mentioned above separation at the porous Si layer is not always made successfully. Therefore, there are many cases in which a crack is generated in the epitaxial layer, thus lowering the yield. Also, since the process achieves the separation by pulling the porous Si layer, a strong adhesion is required between the jig and the single crystal Si layer, which makes the process unsuitable for mass production.